This invention relates to high intensity arc discharge lamps and more particularly to high intensity ceramic metal halide lamps.
Due to the ever-increasing need for energy conserving lighting systems that arc used for interior and exterior lighting, lamps with increasing lamp efficacy are being developed for general lighting applications. Thus, for instance, electrodeless fluorescent lamps have been recently introduced in markets for indoor, outdoor, industrial, and commercial applications. An advantage of such electrodeless lamps is the removal of internal electrodes and heating filaments that are a life-limiting factor of conventional fluorescent lamps. However, electrodeless lamp systems are much more expensive because of the need for a radio frequency power system which leads to a larger and more complex lamp fixture design to accommodate the radio frequency coil with the lamp and electromagnetic interference with other electronic instruments along with difficult starting conditions thereby requiring additional circuitry arrangements.
Another kind of high efficacy lamp is the arc discharge metal halide lamp that is being more and more widely used for interior and exterior lighting. Such lamps arc well known and include a light-transmissive arc discharge chamber sealed about an enclosed a pair of spaced apart electrodes. This chamber typically further contains a chamber materials composition of suitable active materials such as an inert starting gas and one or more ionizable metals or metal halides in specified molar ratios, or both. They can be relatively low power lamps operated in standard alternating current light sockets at the usual 120 Volts rms potential with a ballast circuit, either magnetic or electronic, to provide a starting voltage and current limiting during subsequent operation.
Such lamps may more particularly have a ceramic material arc discharge chamber that usually contains a chamber materials composition having quantities of sodium iodide (NaI), thallium iodide (TlI) and rare earth halides such as dysprosium iodide (DyI3), holmium iodide (HoI3), and thulium iodide (TmI3) along with mercury (Hg) to provide an adequate voltage drop or power loading between the electrodes. Lamps containing those materials have good performance with respect to Correlated Color Temperature (CCT), which lamps typically exhibit relatively lower correlated color temperatures of 2700K to 3700K, and to Color Rendering Index (CRI), and which also have a relatively high efficacy, up to 95 lumens-per-Watt (LPW) when operated at rated power of 150 W. Of course, to further save electric energy in lighting by using more efficient lamps, high intensity arc discharge metal halide lamps with even higher lamp efficacies are needed.
Also, further savings of electrical energy can be had by dimming such lamps during use when full light output is not needed through reducing the electrical current therethrough, and so high intensity arc discharge metal halide lamps with good performance under such dimming conditions are desirable for many lighting applications. However, under these dimming conditions when lamp power is reduced to about 50% of rated value, the performance of currently available lamps of this kind deteriorate significantly. Typically, the correlated color temperature increases significantly, while the color-rendering index (CRI) decreases. Furthermore the efficacy of the lamp usually decreases significantly.
In addition, the lamp hue will deteriorate under such dimming conditions from white to greenish depending on the chemistry. That is, such ceramic material chamber arc discharge metal halide lamps radiate light in which the color rendering index decreases significantly through having a strong green hue due to relatively strong thallium radiation at its characteristic spectral green lines of wavelength 535.0 nm. The discharge tube wall temperatures as well as its cold-spot temperature are much lower at dimming compared to the corresponding temperatures at rated power. At the lower cold-spot temperature occurring under dimming conditions, the ratio of partial pressure of thallium iodide, or TlI, in the discharge tube is much higher compared to the partial pressures of other metal halides leading to this relatively higher TlI partial pressure causing relatively stronger green Tl radiation at the wavelength 535.0 nm. Since the Tl radiation at 535.0 nm is very close to the peak of the human eye sensitivity curve, however, higher lumen efficacy is achieved at rated lamp power with TlI as one of the discharge tube filling components so that it is used in almost all typical commercially available ceramic metal halide lamps.
One possible way of removing the greenish hue under dimming conditions is to remove TlI from the arc discharge chamber altogether and substitute therefor another active material such as PrI3. Another way is to have the arc discharge tube contain halides of Mg, Tl and one or several of the elements from the group formed by scandium (Sc), ytterbium (Y) and lanthanum (Ln). Magnesium iodide, or MgI2, is included as an addition to improve lumen maintenance through influencing the balance of one or several chemical reaction between Sc, Y and Ln and spinel (MgAl2O4) to such an extent that this balance is achieved shortly after the beginning of the lamp operating life after which further removals of Sc, Y and Ln do not take place. Since the Mg addition through MgI2 is for reducing chemical reaction between the chamber materials composition components and the chamber wall, the quantity of MgI2 used in chamber materials composition components in this arrangement is based on the surface area of the inner wall of the discharge vessel.
The arc discharge tube in this last described arrangement is operated within an evacuated outer envelope to reduce convection heat loss from the cold spot of the discharge chamber, and with a metal beat shield used on the discharge chamber to reduce radiation heat loss from the cold-spot during dimming because of the thermal emissivity of the metal shield being much lower than that of the arc discharge chamber ceramic surface, and because of the emissivity of the metal going down as the temperature drops thereby keeping the chamber cold spot temperature and the vapor pressure of the salts in the chamber substantially constant. However, such a lamp still has the disadvantage of radiating with a relatively strong green hue when dimmed to lower than the rated power due to the relatively higher vapor pressure of TlI under dimming conditions, and the further disadvantage that the widely used high voltage starting pulses on low wattage metal halide lamps, when used in conjunction with a vacuum envelope, may make the lamp susceptible to arcing if the discharge tube leaks or slow outer jacket leaks exist. Thus, there is a desire for arc discharge metal halide lamps having higher efficacies and better color performance under dimming conditions.
The present invention provides an arc discharge metal halide lamp for use in selected lighting fixtures having a discharge chamber with electromagnetic radiation or visible light permeable walls of a selected shape bounding a discharge region through which walls a pair of electrodes are supported in the discharge region spaced apart from one another. Ionizable materials are provided in the discharge region of the discharge chamber comprising mercury, a noble gas, and at least two metal halides including a magnesium halide and a sodium halide, a rare earth element, and thallium iodide in a molar quantity which is between 0.7 and 5% of that total molar quantity of all halides present in the discharge chamber.
The discharge chamber can have walls formed of polycrystalline alumina among other materials, and is enclosed in a visible light permeable bulbous envelope positioned in a base with electrical interconnections extending from the discharge chamber to the base and contains a nitrogen gas atmosphere. A shroud of a visible light permeable material can be provided about the discharge chamber. The ionizable materials can further include halides of a series of rare earth elements comprising dysprosium, holmium, thulium, cerium, praseodymium, scandium, neodymium, europium, lutetium and lanthanum so that the total molar quantity of such halides along with the metal halides present in said discharge chamber is between 95 and 99.3% of that total molar quantity of all halides present in said discharge chamber.